Closed head trauma has been understood mostly by physicians looking over surgical findings, radiologic imaging studies and autopsy series of deceased human beings. It has been described as ‘coup-contrecoup’ injury or ‘acceleration-deceleration injury’, based on location of damaged brain tissues and an obvious sequence of events of a sudden forward movement of the brain toward an impact followed by a bouncing-back recoil of the brain. There has been a good deal of consensus as to how an injury to a direct impact site of a brain tissue would occur, but theories are abound to explain mechanisms of the contrecoup injury to the brain tissue. As of 2016, these include ‘positive pressure theory’, ‘rotational shear stress theory’, ‘angular acceleration theory’, ‘cerebrospinal fluid displacement theory’ and ‘negative pressure theory’. Yet none of these theories have been able to propose and verify unifying mechanisms of the brain injury of the so-called contrecoup injury and other associated injuries. In addition, there has not been a validation of a cause and an effect on a series of cases of chronic traumatic encephalopathy sustained by many combat soldiers and athletes. It stands to reason that we may have been blindsided by anatomic findings of the injury and the intricate nature and complex composition of the human head.
Injury to a human tissue can be understood by principles of mechanical waves in physics. A prime example of this which has been utilized for diagnostic purpose of human diseases over many decades is ultrasonographic evaluation of the human tissue. An ultrasound probe emits a range of ultrasonographic waves that are transmitted through the human tissue and a part of the ultrasonographic waves are reflected upon each tissue back to the ultrasound probe. The reflected ultrasonographic waves are registered by the ultrasound probe, which then are electronically interpreted to produce visualized images. Principles of ultrasonographic imaging technique essentially follow the principles of mechanical waves in physics, with the mechanical waves for the ultrasonographic imaging being ultrasound waves. What it means is that no matter how complex and intricate the human tissue would be, the human tissue is no exception for understanding consequences of a delivery of mechanical waves to said human tissue. An impact of a trauma to the human body should be understood as the delivery of mechanical waves to the human body which then undergoes intercellular and intracellular changes including macro- and micro-structural changes. Changes in electrochemical, molecular and signaling pathways of the tissue must occur, but as of now, we are at an early stage of our understanding of pathogenesis of the trauma and its consequences.
One of the extensively studied mechanical waves starting with a sudden big impact energy is seismologic waves which primarily consist of body waves traveling the earth's inner layer and surface waves rippling across the surface of the earth. The body waves comprises P (primary) waves which behave like sound waves and compress objects along their path, and S (secondary) waves which move through solid materials but not liquid materials. Of various physical properties of both the P and S waves, the boundary effects of the waves and the transfer function for the waves would potentially be important for the pathogenesis of the injury to the brain as the human head is a multi-layered structure consisting of several tissues with each having a distinctively different physical property. Layered from a surface to a deep portion of the brain, the head consists of skin and soft tissue underlying the skin, skull, dura mater, arachnoid membrane, leptomeninges and brain tissue proper in sequence. Inside the brain tissue proper, there are blood vessels and fluid sacs named as ventricular space lined by the leptomeninges.
All of these tissues would be damaged simultaneously in an instant without differences in a degree of the damage if the human head sustains a blunt trauma that has P and S waves having an amplitude and a frequency exceeding a tolerability limit of all of the tissues of the human head. However, there would be differences in the degree of the damage to each tissue of the human head if the amplitude and frequency of the P and S waves of the blunt trauma are within the tolerability limit of the tissues. Upon a blunt trauma to the human head which has an amplitude and a frequency of the P and S waves within the tolerability limit of the tissues, presence of a collected liquid in the head such as in blood vessels and ventricles and differences in proportion of liquid content of the tissues would play a role by the transfer function of medium in differences in the degree of the damage to the tissues of the human head. The amplitude of the P and S waves of the blunt trauma may be amplified or deamplified based on a transfer function of the blood vessels, ventricles and a liquid content of the brain tissue proper. In the field of ultrasonographic imaging of human tissue, it is a well-known phenomenon to obtain an augmented amplitude of reflected ultrasound waves back from a tissue behind a fluid sac, which is called acoustic enhancement. It is conceivable to anticipate such amplification of the P and S waves from a tissue behind large sized blood vessels and ventricles located in a relatively linear path from an original site of the blunt trauma on the human head. It is intriguing to note that two of the most common sites of the chronic traumatic encephalopathy are thalamus and amygdala just below the fluid filled lateral and third ventricles of brain, which suggests that an amplitude of the mechanical waves of an impact on a frontal or a vertex portion of a skull coming to the thalamus and the amygdala via the lateral and thrid ventricles may be amplified by presence of a cerebrospinal fluid inside the lateral and third ventricles by a mechanism of the transfer function of a medium similar to the acoustic enhancement of the ultrasonographic imaging.
The surface waves which is known to ripple across the surface of the earth would also be applicable to our understanding of the pathogenesis of the injury to the human head as the brain is relatively spherically round in configuration and encased by the skull which serves to contain the brain in a bowl configuration. Upon a blunt trauma to the human head which has an amplitude and a frequency of the Love waves and the Rayleigh waves of the surface waves within the tolerability limit of the tissues, both the brain and skull may develop resonant amplification of the surface waves, increasing a damage potential of the blunt trauma to the brain.
Both the boundary effects of and transfer function for the P and S waves of the blunt trauma would be useful for mitigating the injury to the brain tissues. If both the P and S waves of the blunt trauma run into a single boundary generated by a single dividing layer inside a protective shell for the human head at an angle, which is understood as a fixed end for the boundary effects in physics term, there is no displacement at the single boundary of the single dividing layer inside the protective shell but stress (amplitude) of the P and S waves on the single boundary of the single dividing layer of the protective shell is known to be temporarily doubled from the original stress of the P and S waves as long as the P and S waves are maintained within the shell. If the P and S waves are released from the shell upon an impact on the single boundary of the single dividing layer of the shell, an amplitude of the P and S waves on the single boundary of the single dividing layer is to be proportionally reduced. If the protective shell has two boundaries, incident P and S waves to the first boundary of the first dividing layer will be both reflected back and transmitted to the second boundary of the second dividing layer. Similarly, a part of the P and S waves will be reflected from the second boundary of the second dividing layer, heading back to an opposite side of the first boundary of the first dividing layer, and the other part will be transmitted to the brain tissue. The reflected P and S waves from the second boundary of the second dividing layer will collide at the first boundary of the first dividing layer with another P and S waves bouncing back from an original site of the blunt trauma toward the first boundary of the first dividing layer, thus neutralizing at the first boundary of the first dividing layer the amplitude of stress from the P and S waves from both the second boundary of the second dividing layer and the original site of the blunt trauma to an extent. If the P and S waves on to the second boundary of the second dividing layer are released from the shell upon the impact much the same way as the P and S waves on to the first boundary of the first dividing layer are released, an overall amplitude of the P and S waves to the second boundary of the second dividing layer will be accordingly reduced. If there are multiple boundaries and the P and S waves are released upon their impact on each boundary of a dividing layer before the P and S waves get to the brain tissue, the amplitude of the P and S waves to the brain tissue will be reduced proportionally to the number of the boundaries of the dividing layers.
A transfer function of a medium for P and S waves depends on fundamental frequency of the medium, which may amplify or deamplify the P and S waves coming from a source. Of solid materials, rigid elastic materials, liquid materials and gaseous materials, the gaseous materials such as air have the lowest fundamental frequency. If the P and S waves from the original site of the blunt trauma go through a gas medium before reaching the brain tissue, these waves will be deamplified resulting in a decrease in an amplitude of the waves to the brain tissue.
Resonant amplification of the surface waves rippling through the protective shell and the human head should also be deamplified as the surface waves in phase with the P and S waves would amplify the P and S waves, increasing the damage potential of the blunt trauma. One way of reducing the resonant amplification of the surface waves is to use the free-end boundary effect at a circular rim end of each boundary of a dividing layer inside the protective shell. At the circular rim end of the boundary of the dividing layer which is free-ended in physics term, traveled waves from the blunt trauma generate zero stress to the circular rim end but displacement of the circular rim end is temporarily doubled. If the free-ended circular rim end is made displaced freely without reflecting back or transmitting the surface waves to other boundaries, the free-ended circular rim end of the boundary of the dividing layer will oscillate on its own upon arrival of the traveled surface waves without further amplification.
Intensity of an amplitude of the mechanical waves delivered to the brain tissue depends on a mass (weight) of a source generating the mechanical waves multiplied by a velocity of an impact from the source and a mass (weight) of a victim and a stopping distance of the impact by the victim colliding with the source: KE=½×mv2 where KE is kinetic energy before an impact, m is mass in kg and v is velocity in meter/second. Since the stopping distance of the impact by the victim is a relatively fixed value (a head does not fall off from a body) and the velocity of the impact from the source could be a relatively fixed value depending on a type of collision, for an example in a collision during a close body fighting sequence, the weight of both the source and victim for the most part would determine the amplitude of the mechanical waves from the impact. What this suggests is that a one-size-fits-all protective headgear is not proper for a group of human beings with a range of different body weights. A person with a lighter body weight as a source of an impact of a blunt trauma on the other person will incite a less powerful amplitude of mechanical waves of the impact than a person with a heavier weight. By the same token, a person with a heavier weight as a victim of a blunt trauma to the head may not be protected well by a protective headgear which is known to protect a person with a lighter weight. Different types of an impact of the blunt trauma would change the velocity of the source of the impact and of the victim. For examples, a collision of a professional bicyclist at a high speed to a stationary object such as a utility pole on street should be different from two football players wrestling with each other and abutting each other's head.
There are two methods to reduce the amplitude of the mechanical waves delivered to the brain tissue, using the multi-layered protective shell with the aforementioned principles: one method is to increase the number of the boundaries inside the protective shell as practically many as possible to a point there would not be a serious tissue injury to the brain tissue; the other is to pressurize the protective shell with a gas and to let the gas released upon an impact from the blunt trauma. If an amplitude of mechanical waves of a blunt trauma does not exceed a resistive pressure of an impacted gas inside the protective shell, the amplitude of the mechanical waves will go through the layered boundaries in the way described above except that the impacted gas would not be released and some of the mechanical waves will transform to heat and some others transmitted to the brain tissue. If the amplitude of the mechanical waves of the blunt trauma exceeds the resistive pressure of the impacted gas inside the protective shell, then a portion of the impacted gas will be released from the protective shell upon the impact of the blunt trauma. It results in a depletion of a portion of an impact energy carried in the impacted gas, which is a decrease in the amplitude of the mechanical waves reaching the brain tissue. While the number of the layered boundaries of the protective shell is fixed once manufactured, the pressure of the gas in the protective shell can be variably adjustable based on a weight of a person wearing the protective shell and anticipated types and scenarios of an injury. Combining both methods for the protective shell would therefore be more advantageous to using either method alone.